1. Field of the Invention
This invention relates to a nitride compound semiconductor laser (hereafter, may be simply referred to device) having a plurality of crystal layers of group III nitride compound semiconductor expressed by the formula (AlxGa1-x)1-yInyN (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), to which carriers are supplied from electrodes. More specifically, this invention relates to a group III nitride compound semiconductor laser that can emit laser light of wavelengths ranging from ultra-violet to blue and to a method for manufacturing the same.
2. Description of Related Art
A number of possible structures for semiconductor laser have been proposed over the years. Many of them include structures for limiting the area for current injection in the direction parallel to the pn junction, namely transverse direction, and those for confining light generated in the active layer in the transverse direction. Those structures typically break down into two types: the ridge-type, namely mesa-stripe type; and the inner stripe type, namely the internal current flow restriction type.
The ridge-type semiconductor laser has the so-called ridge-type structure in which a stripe-shape narrow bump is formed in a region above a p-type guide layer and a p-side electrode is formed on this bump. The device of this type needs a high-precision process of the ridge structure. This process consists of numerous steps and makes it difficult to improve the manufacturing yield of the device. This is because the dimensions of the ridge structure significantly influence the threshold current for oscillation and light-beam quality.
Meanwhile, Japanese Patent Kokai No. Hei. 11-261160 discloses a group III nitride compound semiconductor laser of an inner stripe type that has a pair of clad layers, an active layer sandwiched between the clad layers and a current constricting layer having a stripe-shape aperture serving as the path for current over the active layer. This current constricting layer is a highly resistant layer that is fabricated by heating an amorphous or poly-crystalline nitride compound semiconductor layer and then crystallizing this layer. This current constricting layer is made of GaN containing impurities at least 1xc3x971020 cmxe2x88x923. The light is confined in the transverse direction by utilizing the light absorption effect relevant to the impurity energy states in this layer.
However, the clad layer over the aperture of the current constricting layer is regrown on the uneven (bumpy) underlying layer. As a result, when the p-type nitride compound semiconductor containing group II Mg as an acceptor impurity is regrown on the current constricting layer, the distribution of Mg concentration is not uniform in the semiconductor layer of the aperture and then its electric performance deteriorates.
In the case of nitride compound semiconductors, the satisfactory p-type conduction is realized when the Mg concentration is within a very limited range. Thus if there are fluctuations in the distribution of Mg concentration, the p-type conduction properties deteriorate.
In particular, when a p-type clad layer, which is usually AlxGa1-xN:Mg (0.05xe2x89xa6xxe2x89xa60.20), is regrown, an inhomogeneous distribution of Mg in the semiconductor layer of the aperture exerts seriously negative effects. That is, a potential barrier to an injection of carriers (in this case, holes) is developed unless the clad layer itself is a uniform p-type layer since the band gap of the clad layer is larger than that of the guide layer. Besides, the series resistance of the device increases due to the rise in the bulk resistance of the p-type AlGaN. Namely, if the semiconductor layer filling the aperture is the Mg-doped AlGaN clad layer, a degradation of the p-type conduction in this layer directly affects the current-voltage properties of the resulting device.
Operating current and voltage can be lowered in the inner stripe type laser since it provides both the restriction of current injection area and the light confinement in the transverse direction at the same time. It shows good performance in controlling the transverse mode of light and may be manufactured at a high productivity. Compared with the ridge type laser, the inner stripe type laser shows better performance in heat dissipation, providing a long life of use and high reliability. Despite these merits, as the aforementioned problems have not been solved, the inner stripe type semiconductor laser using group III nitride compound semiconductor is not popular yet. Only the ridge type group III nitride compound semiconductor laser has been successfully commercialized so far.
This invention has been made to solve the problem that the conventional inner stripe type nitride compound semiconductor laser has poor current-voltage properties. An object of the present invention is, therefore, to provide an inner stripe type nitride compound semiconductor laser that can be driven at low current and voltage, easy to manufacture and stable during operation at the transverse mode of light.
The present invention provides a group III nitride compound semiconductor laser that has a pair of opposing guide and clad layers sandwiching an active layer and a current constricting layer located intermediate in a p-type guide layer.
The current constricting layer is made of AlN deposited at low temperatures between 400-600xc2x0 C. and has a stripe-shape aperture that restricts the area through which current is injected to the active layer. Namely, the nitride compound semiconductor laser according to the present invention is a nitride compound semiconductor laser having a plurality of crystal layers made of a group III nitride compound semiconductor expressed by the formula (AlxGa1-x)1-yInyN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), the plurality of crystal layers comprising an active layer-side guide layer which is adjacent to the active layer in the crystal layers of the group III nitride compound semiconductor and made of Alxxe2x80x2Ga1-xxe2x80x2-yxe2x80x2Inyxe2x80x2N (where 0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6yxe2x80x2xe2x89xa61), a current constricting AlN layer which is deposited on said guide layer and has a stripe-shape aperture, an electrode-side guide layer which is made of Alxxe2x80x3Ga1-xxe2x80x3-yxe2x80x3Inyxe2x80x3N (where 0xe2x89xa6xxe2x80x3xe2x89xa61, 0xe2x89xa6yxe2x80x3xe2x89xa61) and deposited filling the aperture of the current constricting layer, and a clad layer which is made of AluGa1-u-vInvN (where 0xe2x89xa6uxe2x89xa61, 0xe2x89xa6vxe2x89xa61) and deposited on the electrode-side guide layer. The current constricting layer can block current effectively in the regions other than the aperture because the electric resistance of the AlN film deposited at low temperatures (400-600xc2x0 C.) is very high.
In the nitride compound semiconductor laser according to the present invention, if the band gaps of the active layer-side guide layer, the electrode-side guide layer and the clad layer are represented by Eg1, Eg2 and Eg3, respectively, their relations are Eg1xe2x89xa6Eg2xe2x89xa6Eg3.
The guide layer on the active layer side and the guide layer on the electrode side may have the same composition, AlxGa1-x-yInyN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
A semiconductor layer of AlzGa1-zN (where 0.05xe2x89xa6zxe2x89xa60.3) may be formed immediately above the active layer of the device in order to protect the active layer and prevent the overflow of electrons.
The film thickness of the current constricting AlN layer is 100-800 xc3x85, preferably 200-600 xc3x85 in the present invention. Since the refractive index of the AlN layer is approximately 2.15 and smaller than those of the other regions, an effective step of the refractive index is provided that can confine light in the horizontal direction (transverse direction) parallel to the pn junction in the vicinity of the aperture of the current constricting layer.
If the AlN film becomes thinner than the lower limit, 100 xc3x85, it becomes difficult to effectively confine the light in the transverse direction. The light confinement in the transverse direction according to this invention is different from the conventional one utilizing the current constricting layer where impurities have been heavily doped to provide an appropriate absorption coefficient for the light emitted from the active layer. The present invention utilizes an effective step of the refractive index resulting from an appropriate low refractive index of AlN.
The AlN film deposited at low temperatures is amorphous (non-crystal) and this layer is used as a buffer layer that relaxes mismatching in the lattice constant when growing GaN on the sapphire substrate. Therefore, it is easy to regrow GaN:Mg layer(p-type guide layer) on the low temperature deposited AlN layer. If the AlN film is thicker than the upper limit, 800 xc3x85, it becomes almost impossible to sufficiently flatten by the deposition of the electrode-side guide layer. Group II elements such as Mg are added to the guide layer, and the guide layer comes to show p-type conduction through annealing treatment.
In the present invention, the current constricting layer made of AlN is located inside the p-type guide layer. Thus this structure can avoid the conventional problem that the current-voltage properties deteriorate due to the increase of the electric resistance of the p-type crystal layer within the aperture of the current constricting layer.
The Mg distribution in the p-type crystal layer within the aperture of the current constricting layer becomes inhomogeneous during the growth of the p-type guide layer as well. However, the resulting current-voltage properties of the device are good. This is because the band gap of the guide layer is designed to be smaller than that of the clad layer and a large number of carriers (in this case, holes) can flow into the guide layer from the clad layer to provide a high conductivity for the guide layer when the device is forward-biased. Therefore, if the clad layer is a good p-type layer, a sufficient number of carriers are supplied from the clad layer. This explanation is consistent with the fact that laser oscillation is realized even if the guide layer is undoped. Besides, when the aperture is buried with this p-type guide layer made of GaN:Mg, the inhomogeneity in Mg incorporation works preferably, and a flat surface of buried layer is provided. As a result, the p-type contact layer and the electrode that will be deposited over this buried layer become flat as well. Then the semiconductor laser of the inner stripe type according to the present invention provides another merit that it is easy to attain a thermally preferable contact when mounting the device on a heat sink with its p-side down.
Moreover, AlN has a merit in terms of heat dissipation because it has a thermal conductivity of 285W/mK around at room temperature, which is more than twice the thermal conductivity of GaN, 130 W/mK. This feature of AlN is a merit whether the device is mounted on the heat sink with the p-side up or p-side down, thus contributing to a longer life of use.
A manufacturing method according to the present invention for a nitride compound semiconductor laser having a plurality of crystal layers made of a group III nitride compound semiconductor expressed by the formula (AlxGa1-x)1-yInyN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), comprising the steps of forming an active layer-side guide layer that is adjacent to the active layer made of the group III nitride compound semiconductor and made of Alxxe2x80x2Ga1-xxe2x80x2-yxe2x80x2Inyxe2x80x2N (where 0xe2x89xa6xxe2x80x2xe2x89xa61, 0xe2x89xa6yxe2x80x2xe2x89xa61), forming a current constricting AlN layer that is deposited on said active layer-side guide layer and has a stripe-shape aperture, filling said current constricting layer with an electrode-side guide layer made of Alxxe2x80x3Ga1-xxe2x80x3-yxe2x80x3Inyxe2x80x3N (where 0xe2x89xa6xxe2x80x3xe2x89xa61, 0xe2x89xa6yxe2x80x3xe2x89xa61), and forming a clad layer made of AluGa1-u-vInvN(0xe2x89xa6uxe2x89xa61, 0xe2x89xa6vxe2x89xa61) on the electrode-side guide layer. The current constricting layer made of AlN can be etched by wet process and it is easy to form the aperture. With respect to this feature, the device manufacturing method disclosed in Japanese Patent Kokai No. Hei. 11-261160 describes that the etching becomes difficult to perform when forming a GaxAlyIn1-x-yN(0xe2x89xa6x, yxe2x89xa61) film if its Al content is high because of its higher etching-resistance. However, we have found that, contrary to this disclosure, it is possible to easily wet-etch AlN that was deposited at temperatures between 400-600xc2x0 C. with an alkaline solution kept at appropriately 80xc2x0 C. or phosphoric acid-base etching solution kept at 150-200xc2x0 C. The wet etching of AlN formed on the p-type GaN provides a highly clean surface. Thus when the p-type guide layer is regrown after the aperture formation, the interface resulting from this film regrowth does not work as a barrier that contains too many impurities. There is no degradation in the current-voltage properties of the device. The growth of these crystal layers is conducted by the metalorganic chemical vapor deposition method.